This invention relates to a method of protecting steel sheet with a nontoxic, relatively insoluble, inorganic, corrosion resistant coating. More particularly, the invention relates to steel sheet having a silicate coating formed from an aqueous alkaline waterglass solution with the silicate coated sheet treated with an aqueous silane containing solution.
Terne, i.e., lead and lead alloy containing tin, coated steel sheet has long been plagued with the problem of red rusting of the steel substrate. This rusting is caused by poor wetting of the substrate by the coating metal resulting in pin holes or uncoated areas. Red rusting also may be caused by fine iron particles contained in the terne coating metal. There have been numerous proposals to minimize this corrosion by sealing the pin holes with inorganic coatings such as phosphate; chromate; alkali salts of borate, silicate, benzoate or carbonate; zinc sulphite; zinc nitrate; sodium fluoride; and various acidic solutions including sulfuric, phosphoric, hydrochloric, boric, carboxylic, iminodiacetic and nitrilotriacetic. Others have proposed applying organic coatings including epoxy, phenolic, polyester, phthalic acid, fluorine, and silicone to seal the pin holes. These coatings generally have been unsatisfactory because they give only limited corrosion protection, are relatively soluble and/or result in a toxic waste disposal problem. Still others have proposed electroplating a thin nickel coating onto the steel sheet prior to plating the sheet with terne metal. This nickel-terne composite coating has reduced the occurrence of pin holes but has done little for red rusting caused by the corrosion of the fine iron particles contained in the terne metal.
Phosphate conversion coatings are widely used on automotive steels to improve the corrosion performance of painted steel sheets. The corrosion processes of painted steel usually involve high pH conditions at the paint-metal substrate interface. Since phosphate coatings are unstable in an alkaline environment, phosphated steels are rinsed with solutions containing chromium or chromate ions to improve their alkaline stability. However, recent studies suggest the improvement is marginal. Although dry paint adhesion on chromated phosphated steel sheet is good, wet paint adhesion is unacceptable. The bond between the paint-phosphate interface is weak when water or other corrosion species are present.
Chromate coatings have been used to improve corrosion resistance of cold-rolled steel by minimizing red rusting and of galvanized steel by minimizing white rusting. Unfortunately, hexavalent chromium has carcinogenic properties. Because of their toxic nature, rinses containing chromate ions are undesirable for industrial usage.
It also has been proposed previously to improve corrosion resistance and paint adhesion of cold-rolled or galvanized steel sheet using organic polymeric coatings containing a silane and using inorganic coatings including a silane and a silicate.
It also has been proposed to improve alkaline corrosion resistance and paint adhesion of phosphated cold-rolled or galvanized steel sheet using a two step process including rinsing the sheet in an alkaline waterglass solution to form a silicate coating and subsequently rinsing the silicate coated sheet in an aqueous silane containing solution. The sheet first was rinsed for a period of 30 minutes in a 0.005M waterglass solution maintained at room temperature and having a pH of 12. After being blown dry, the silicate coated sheet was rinsed for 5 minutes in 0.5 vol.-% silane solution. The silane solution was prepared by dissolving either .gamma.-aminopropyltri(m)ethoxy (APS) or .gamma.-glycidoxypropyltrimethoxy (GPS) silane in an aqueous solution containing acetic acid. The silane treated silicate coating improved the alkaline resistance of phosphate coatings on cold-rolled and galvanized steel, improved the adhesion between the phosphate crystals and paint primer and improved the corrosion resistance of painted steel. Nevertheless, this proposed process has limited industrial application because of the excessive immersion times. Combined immersion times in excess of 30 minutes to apply the silicate coating and the silane treatment are unacceptable for steel sheets which require high speed processing to be cost competitive.
As evidenced by the effort of previous workers, there has been a long felt need to develop a low cost, nontoxic, relatively insoluble, corrosion resistant coating for a steel sheet that is formed using environmentally safe coating solutions that can be disposed of inexpensively. There also has been a need for such a nontoxic coating for preventing corrosion of a terne coated steel sheet. Furthermore, there has been a need for such a nontoxic coating for preventing corrosion and improving paint adherence of cold-rolled or galvanized painted steel sheets.